The present invention relates generally to a resin-coated composite foil. More particularly, the invention relates to a resin-coated composite foil which can suitably be employed in the production of high-density printed wiring boards, a process for producing the resin-coated composite foil, and processes for using the foil in producing multilayer copper-clad laminates and multilayer printed wiring baords.
Laminates for printed wiring board used in electronic materials are commonly produced by impregnating a glass cloth, a kraft paper, a glass nonwoven fabric or the like with a thermosetting resin such as a phenolic resin or an epoxy resin, semicuring the thermosetting resin to thereby obtain a prepreg and laminating one side or both sides of the prepreg with a copper foil. Further, multilayer printed wiring boards are commonly produced by forming wirings on both sides of a copper-clad laminate to thereby obtain an inner core material and laminating both sides of the inner core material, through the medium of prepregs, to additional copper foils.
In recent years, it has been common to furnish multilayer printed wiring boards with small holes between layers, i.e., via holes in conformity with the increase of the density of printed wiring board. Such via holes can be formed by, for example, laser beams or plasma machining. Results with laser beams or plasma machining are poor when prepregs containing an inorganic component such as glass fiber are used as an insulation layer. Therefore, only resins containing no inorganic component are increasingly used for the insulation layer. Accordingly, for example, a resin film composed of a semicured thermosetting resin or a resin-coated composite foil obtained by applying a resin to one side of a copper foil and semicuring the resin is used for the insulation layer.
Printed wiring boards are produced by laminating such a resin film or resin-coated composite foil onto a printed wiring board furnished with wirings (inner core material) and then forming wirings and via holes. The thus obtained laminates possess heat resistance, electrical properties and chemical resistance that are satisfactory in practical use as printed wiring board.
Although the current copper foil used in the resin-coated copper foil is generally an electrolytic copper foil having a thickness of 12 to 35 xcexcm, the use of a thinner copper foil is required when it is intended to provide more minute wirings, i.e. having very small wiring lines and spaces. However, resin-coated copper foils produced by applying a resin varnish to an ultra-thin copper foil having a thickness of 12 xcexcm or less and heating and drying the same have various drawbacks.
For example, it is very likely for the copper foil to be broken during the coating, heating or drying step, thereby rendering stable production difficult. Another problem is that the applied resin layer shrinks during the drying step to thereby increase the likelihood of deformation of the resin-coated copper foil, namely curling thereof, with the result that the handling of the resin-coated copper foil is very difficult. A further problem resides in that the resin composition for use in the resin-coated copper foil must be as proposed by the inventors (Japanese Patent Application No. Hei 9-176565) in order to prevent cracking of the resin layer, thereby restricting the resin blend formulation. There is still a further problem such that, when ultra-thin copper foils and inner wirings are combined to construct a multilayer board, the ultra-thin copper foils are broken or wrinkled by the unevenness of a surface of the inner wirings.
The method of interposing a thick copper foil or plastic film between a hot press plate and a resin-coated copper foil during the laminating step is known as a countermeasure to the above problems. Furthermore, the method of producing a resin-coated composite foil from an ultra-thin copper foil furnished with a supporting metal foil (carrier) has been proposed as described in Japanese Patent Application Publication (Unexamined) No. Hei 9-36550. Generally, either the etchable type having its support metal foil selectively removed with the use of a liquid chemical or the peelable type having it support metal foil mechanically stripped is used as the above ultra-thin copper foil furnished with the supporting metal foil.
However, the above method of interposing the thick copper foil or plastic film between the press hot plate and the resin-coated copper foil during the laminating step has drawbacks in that the cost of copper foil and plastic film are incurred and that the working efficiency is deteriorated. Further, when a plastic film is interposed, the plastic film is charged with static electricity so that dust in the working environment is likely to deposit on the surface of the plastic film. Thus, the dust is transferred to a product to thereby bring about etching failure or other problems. Moreover, the conventional production of a resin-coated composite foil from an ultra-thin copper foil furnished with the supporting metal foil has also drawbacks. Specifically, the use of the etchable type carrier creates problems in that the number of process steps is increased by the etching and disposal of etching waste liquid is required. On the other hand, the use of the peelable type carrier creates problems in that it is difficult to optimize the bonding strength between the supporting metal foil and the ultra-thin copper foil. That is, when the bonding strength is too low, although the stripping of the supporting metal foil after lamination onto a base material is facilitated, peeling is likely to occur between the supporting metal foil and the ultra-thin copper foil when, while applying the organic insulation layer, heating and drying is carried out after the application of a resin varnish. Thus, blistering of the ultra-thin copper foil and separation of the supporting metal foil and the ultra-thin copper foil from each other is likely to occur and it renders practical production difficult. In contrast, when the bonding strength between the supporting metal foil and the ultra-thin copper foil is increased, although no problems occur during the resin varnish application and heating/drying steps, it has been found that, during the step of stripping the supporting metal foil after the lamination onto the base material, stripping is difficult and the base material is deformed by stress attributed to the stripping with the result that the base material suffers from a residual strain increase and cracking and that inner wirings are broken.
When a laser is employed to provide a copper-clad laminate with via holes, a sodium hydroxide solution is used as a cleaning liquid for removing dust and other matter resulting from layer perforation. This sodium hydroxide solution corrodes the insulation resin, thereby the diameter of via holes formed in the insulation resin layer is larger than desired. On the other hand, a resin composition comprising an epoxy resin blend consisting of an epoxy resin and a curing agent therefor and a thermoplastic resin which is soluble in a solvent and has a functional group, other than an alcoholic hydroxyl group, polymerizable with epoxy resins is available as an alkali resistant resin. However, this resin composition has drawbacks in that, in the B-stage (semicured), cracking is likely to occur in the resin composition and that deformation of resin-coated copper foil during handling thereof is likely to crack the insulation resin layer. In these circumstances, the inventors have made extensive and intensive studies with a view toward solving the above problems. As a result, it has been found that the above technical problems and drawbacks of the prior art can be resolved by disposing an organic insulation layer on an ultra-thin copper foil provided on a supporting metal foil through an organic release layer. The present invention has been completed on the basis of this finding.
An object of the present invention is to solve the above problems of the prior art. An object of the present invention is to provide a resin-coated composite foil which is free from the peeling of the supporting metal foil and the ultra-thin copper foil from each other even during the resin varnish coating and heating/drying steps and which permits extremely easy stripping of the supporting metal foil after the lamination onto a base material. A further object of the present invention is to provide a printed wiring board which has excellent workability in laser and plasma machining and can be made with fine wirings and via holes. Another object of the present invention is to provide processes for producing a multilayer copper-clad laminate and a multilayer printed wiring board with the use of the resin-coated composite foil having high alkali resistance.
The resin-coated composite foil of the present invention comprises:
a supporting metal layer,
an organic release layer disposed on a surface of the supporting metal layer,
an ultra-thin copper foil disposed on the organic release layer, and
an organic insulation layer disposed on the ultra-thin copper foil.
The organic insulation layer is preferably formed from a resin composition comprising:
(i) an epoxy resin blend comprising an epoxy resin and a curing agent therefor, and
(ii) a thermoplastic resin which is soluble in a solvent and has a functional group, other than an alcoholic hydroxyl group, polymerizable with the epoxy resin. This thermoplastic resin is preferably selected from the group consisting of a polyvinylacetal resin, a phenoxy resin and a polyether sulfone resin.
It is preferred that the organic release layer comprises a compound selected from the group consisting of nitrogen-containing compounds, sulfur-containing compounds and carboxylic acids.
The nitrogen-containing compounds are preferably substituted triazole compounds such as carboxybenzotriazole, Nxe2x80x2,Nxe2x80x2-bis(benzotriazolylmethyl)urea and 3-amino-1H-1,2,4-triazole.
Examples of the sulfur-containing compounds include mercaptobenzothiazole, thiocyanic acid and 2-benzimidazolethiol.
The carboxylic acids are preferably monocarboxylic acids such as oleic acid, linolic acid and linolenic acid.
The process for producing a resin-coated composite foil according to the present invention comprises the steps of:
uniformly forming an organic release layer on a supporting metal layer;
electrodepositing an ultra-thin copper foil layer on the organic release layer; and
forming an organic insulation layer on the ultra-thin copper foil layer.
The process for producing a multilayer copper-clad laminate according to the present invention comprises the steps of:
superimposing a resin-coated composite foil (A) comprising a supporting metal layer, an organic release layer disposed on a surface of the supporting metal layer, an ultra-thin copper foil disposed on the organic release layer and an organic insulation layer disposed on the ultra-thin copper foil, and
a copper-clad laminate (B) comprising an insulation base layer having its one side or both sides furnished with inner wiring;
wherein the organic insulation layer of the resin-coated composite foil (A) contacts the wiring furnished side of the copper clad laminate (B), followed by applying heat and pressure to thereby obtain a laminate; and
stripping the supporting metal layer from the laminate.
The process for producing a multilayer printed wiring board according to the present invention comprises forming an outer wiring on the ultra-thin copper foil layer of the multilayer copper-clad laminate produced by the above process for producing a multilayer copper-clad laminate.
The outer wiring can be formed by the steps of forming via holes with the use of UV-YAG laser or carbon dioxide laser, panel plating and etching.